Timken Super Precision Bearings for Machine Tool Applications

A
ENGINEERING
The unique design of a tapered roller bearing with its line contact
produces a damping characteristic that is not necessarily inherent
to other bearing designs (Figure 31). This is due to the bending
mode of the spindle and bearing centerline caused by dynamic
deflection which is resisted inside the bearing through a shearing
action of the viscous lubricant between the rollers and the cup
and cone races.
It is the combination of the tapered roller bearing construction
and proper bearing setting that results in improved damping
characteristics.
Fig. 31. Damping in a tapered roller bearing.
An extension of this insight culminated in the development of a
bearing system called the Hydra-Rib. It is, specifically designed to
provide the optimum bearing preload and thus the ultimate dynamic
stability for the spindle system under any operating conditions.
Proper selection of the preload for a given application must not focus
only on stiffness and damping characteristics. Also, the lubrication
method, operating speeds and loads must be reviewed to determine
the optimum setting/preload to maximize performance. Consult the
appropriate topic in this engineering section for more details.
OTHER FACTORS AFFECTING
BEARING SELECTION
Some additional controllable factors having a significant impact
on bearing performance include mounting fits, internal clearances,
lubricant type and integrity. For example, when using radial ball
bearings, appropriate internal clearance is needed to ensure
proper operation.
THERMAL EXPANSION
Issues such as axial displacement must be addressed where shaft
length differentials must be tolerated when thermal expansion
occurs. For these situations, the rotating component supports
include a fixed (locating) and floating (non-locating) bearing
arrangement.
The fixed bearing is subject to combination loads and is usually
placed nearest the working end of the shaft to minimize motion and
thereby maintain workpiece accuracy. Installation considerations
for the typical fixed bearing positions should note the fitting
suggestions listed in this catalog. These are compiled from a wealth
of experience in a wide range of operating conditions.
Where floating bearings are necessary, the design must allow
for axial displacement of the shaft. This can be accomplished by
allowing the bearing to “slide” laterally along the shaft or housing
respectively. A looser fit during the machining of the shaft or housing
cavity is required. This not only alleviates the axial stresses on
this end of the assembly, but will also facilitate bearing and shaft
installation.
MISALIGNMENT
When the size of the machine increases, so do concerns about
alignment. Shaft bending or additional loading can impart moment
loads that need to be considered. Bearing selection must further
consider installation practices with distant machined bearing
housing cavities. Manufacturing limitations to position housing
bores might encourage the choice of a self-aligning bearing
mounting (DF, face-to-face).
This can help compensate for machining variations and assist in
managing dynamic forces by featuring a spherical outside diameter
or thrust face.
Duplex - DF
Face-to-Face
Duplex
Direct Mounting
Fig. 32. Self-aligning bearing mountings.
34 TIMKEN MACHINE TOOL CATALOG